1. Field of the Invention
The present invention is related to the printing and encoding of labels (or other media) with embedded radio frequency identification (RFID) tags, and more particularly, to printers that ensure the RFID tags are appropriately encoded during label printing.
2. Description of Related Art
UHF radio frequency identification (RFID) technology allows wireless data acquisition and or transmission from and or to active (battery powered) or passive RFID transponders using a backscatter technique. To communicate with, i.e., “read” from and or “write” commands and/or data to an RFID transponder, the RFID transponder is exposed to an RF electromagnetic field by the transceiver that couples with and energizes (if passive) the RFID transponder through electro-magnetic induction and transfers commands and data using a predefined “air interface” RF signaling protocol.
When multiple passive RFID transponders are within the range of the same RF transceiver electro-magnetic field they will each be energized and attempt to communicate with the transceiver, potentially causing errors in “reading” and or “writing” to a specific RFID transponder in the reader field. Anti-collision management techniques exist to allow near simultaneous reading and writing to numerous closely grouped RFID transponders in a common RF electromagnetic field. However, anti-collision management increases system complexity, cost and delay response. Furthermore, anti-collision management is “blind” in that it cannot recognize where a specific RFID transponder being processed is physically located in the RF electro-magnetic field, for example, which RFID transponder is located proximate the print head of a printer-encoder.
One way to prevent errors during reading and writing to RFID transponders without using anti-collision management is to isolate a specific RFID transponder of interest from nearby RFID transponders. Previously, isolation of RFID transponders has used RF-shielded housings and/or anechoic chambers through which the RFID transponders are individually passed for personalized exposure to the interrogating RF field. This requires that the individual RFID transponders have cumbersome shielding or a significant spatial separation.
RFID printers-encoders have been developed which are capable of on-demand printing on labels, tickets, tags, cards or other media with which an RFID transponder is attached or embedded. These printer-encoders have a transceiver for on-demand communication with the RFID transponder on the individual media to read and/or store data into the attached RFID transponder. For the reasons given, it is highly desirable in many applications to present the media on rolls or other format in which the RFID transponders are closely spaced. However, close spacing of the RFID transponders exacerbates the task of serially communicating with each individual RFID transponder without concurrently communicating with neighboring RFID transponders on the media. This selective communication exclusively with an individual RFID transponder is further exacerbated in printers-encoders designed to print on the media in or near the same space as the RFID transponder is positioned when being interrogated.
UHF RFID transponders may operate in, for example, the 902-928 MHz band in the United States and other ISM bands designated in different parts of the world. For example, in FIG. 1 a conventional one-half wavelength “Forward Wave” microstrip prior art coupler 3 consisting of, for example, a rectangular conductive strip 5 upon a printed circuit board 7 having a separate ground plane 9 layer configured for these frequencies. One end of the conductive strip 5 is connected to transceiver 42 and the other end is connected through terminating resistor 8 to ground plane 9. The conductive strip 5 as shown in FIG. 1 has a significant width due to RF design requirements imposed by the need to create acceptable frequency response characteristics. This type of prior art coupler 3 has been used with UHF RFID transponders that are relatively large compared to the extent of prior art coupler 3.
As shown by FIGS. 2a and 2b, recently developed RFID transponders 1, designed for operation at UHF frequencies, have one dimension so significantly reduced, here for example a few millimeters wide, that they will be activated upon passage proximate the larger prior art coupler 3 by electro-magnetic power leakage 10 concentrated at either side edge of the conductive strip 5 of prior art coupler 3. In FIG. 2A, the two leakage regions “A” and “B” defined by electro-magnetic power leakage 10 are small and relatively far apart, increasing system logical overhead and media conveyance positioning accuracy requirements. If the RFID transponders 1 were placed close together, then multiple RFID transponders 1 might be activated by the physically extensive one-half wavelength “Forward Wave” microstrip prior art coupler 3.
Competition in the market for such “integrated” printer-encoder systems as well as other RFID interrogation systems has focused attention on the ability to interrogate with high spatial selectivity any RFID transponder from a wide range of available RFID transponders having different sizes, shapes and coupling characteristics as well as minimization of overall system, media size, and RFID transponder costs. In addition, this high spatial selectivity and wide range of available RFID transponders must be balanced with the need for the integrated printer-encoder system to be able to read and encode RFID transponders of varying configurations at different locations on the media.
The need to read and encode RFID transponders embedded in printer media with high selectivity is addressed by commonly-owned U.S. patent application Ser. No. 10/981,967 entitled “SYSTEM AND METHOD FOR DETECTING TRANSPONDERS USED WITH PRINTER MEDIA” filed on Nov. 5, 2004 which is hereby incorporated herein in its entirety by reference. In this application, a calibration apparatus employs a transceiver that varies its power and a controller that varies the position of the media and RFID transponders to determine the location of the RFID transponders. These locations are then used to instruct communication with the RFID transponders and avoid communicating with closely spaced adjacent RFID transponders.
Despite improvements in the ability of integrated printer-encoder systems to selectively read, encode and otherwise communicate with differently located RFID transponders, the possibility remains that communication will not be established with each RFID transponder. For example, occasional RFID transponders may have defects and therefore be incapable of being written to, or read from, or may result in corruption of the written or read information. In addition, the defective RFID transponders typically interrupt the smooth flow of printing and encoding the labels.
Therefore, it would be advantageous to have a printer-encoder system that is capable of reading and encoding a large number of types of RFID transponders, including closely spaced RFID transponders, and printing on media supporting the RFID transponders, while still ensuring that improperly encoded or defective RFID transponders are discarded or remain unused. In addition, it would be advantageous to have a printer-encoder system equipped to respond to improperly encoded or defective RFID transponders with minimal interruption of printing and encoding operation.